Semiconductor integrated circuit (IC) devices typically employ aluminum as the metal of choice for use with silicon substrates. Typically, an aluminum bonding pad is overlapped with the preferred passivation which is commonly a layer of silicon dioxide overcoated with silicon nitride (Si.sub.3 N.sub.4). The central area of the aluminum bonding pad is left open for electrical connections which will be associated with the IC housing. Typically, bond wires, metal fingers or metal bumps are connected to the bond pads to provide the desired connection. If the metal involved is a noble metal such as gold, which is useful because of its corrosion resistance, it must be isolated from the aluminum by a barrier layer which can be composed of, for example, nickel-alloy, titanium-tungsten alloy, chromium or chromium alloy. Basically, the barrier layer should adhere well to the aluminum and whatever passivation layer is present and it should chemically isolate the aluminum metallization from the noble metal electrical connection.
Typically, in the prior art the aluminum bonding pads exposed inside the passivation layer are overcoated with a succession of metal layers such as aluminum (optional) for initial adherence, a barrier metal layer for isolation and a noble metal for electrical contact. These layers actually overlap the passivation coat so as to completely seal off the original aluminum bonding pad. However, the edges of the metal layers will be exposed to any contaminants that the packaged IC is exposed to. For example, in plastic molded housings the encapsulant is permeable to moisture to some degree. Furthermore, the plastic seal to the metal leads will typically admit moisture along the interface and into the IC, particularly after temperature cycling. Accordingly, it has become common to provide hermetic packaging for devices that require high reliability. While this is effective, it is costly and even hermetic packages have been known to develop leaks. As a result, it is deemed useful to provide a suitable seal for the IC itself wherein any contaminants will not be able to attack the IC components even when they are present inside the package.